This invention is directed to microelectromechanical systems (MEMS) which are used as accelerometers. In particular, this invention is directed to a MEMS accelerometer which achieves differential output using a curved beam
Microelectromechanical systems (MEMS) are devices which may be fabricated using semiconductor thin film technology in order to reduce the characteristic dimensions of the devices. MEMS technology is often applied to the design and fabrication of sensors in particular, because the range of motion in such devices is generally limited, compared to, for example, a motor or actuator. MEMS technology has been applied to the design and fabrication of accelerometers, for example, which detect and measure the presence of accelerative forces.
One example of a prior art MEMS accelerometer is shown in FIG. 1. The MEMS accelerometer 100 may include a beam 130 suspended over a substrate 140 on a fulcrum 150. The beam 130 may include two conductive layers 170a and 170b which may be deposited on the beam 130. Opposite conductive layers 170a and b may be a second pair of conductive layers 180a and 180b, deposited on substrate 140, aligned to correspond to the upper conductive layers 170a and 170b. The beam 130 may also include a proof mass 120, which may render the teeter-totter beam 130 asymmetric, and allow it to respond to the application of an accelerative force 110. The two pairs of conductive layers 170a and 180a, and 170b and 180b may form two pairs of parallel plate capacitors. The top and bottom layers 170a and 180a, and 170b and 180b may have a specific capacitance based on their width, length, and the gap between them. The signal obtained is the capacitance between the top and bottom layers 170a and 180a on left side, and the capacitance between the top and bottom layers on the right side.
In operation, the signal from one set of capacitor plates, for example 170a and 180a, may be subtracted from the signal from the second set of capacitor plates; 170b and 180b. This subtraction may remove sources of DC errors, such as differences in the exact dimensions of the conductive layers, or the, nominal separation between them under zero acceleration conditions. Such differential approaches may be important to improving the accuracy of such accelerometers.
The device shown in FIG. 1 is known as a teeter-totter accelerometer, because a beam 130 pivots on a fulcrum 150. When an acceleration 110 is applied to accelerometer 100, the proof mass 120 causes the beam 130 to rotate clockwise about the fulcrum 150. The beam can pivot on the fulcrum but there is a resistance to movement that is proportional to the angle of rotation. This may be referred to as an angular spring rate. The proof mass 120 that is attached to one end of the beam 130 may apply a force to the end of the beam 130 when the teeter-totter accelerometer 100 is subjected to acceleration 110. The force may cause the beam 130 to rotate in the appropriate direction a distance at which the force of acceleration on the mass equals the resistive force of the spring at the fulcrum 150.
The pivoting motion may cause the gap 175 between the first set of capacitor plates, 170a and l80a, to increase, and the gap 185 between the second set of capacitor plates 170b and 180b, to decrease. Therefore, the capacitance signal Sa from capacitor plates 170aand 180a may decrease by an amount α as a result of the applied acceleration 110, and the capacitance signal Sb from the second set of capacitor plates 170b and 180b, may increase by an equal amount, Δ. Therefore, subtracting the change in capacitance of one set of plates 170a and 180a from the change in capacitance of the second set of capacitor plates 170b and 180b, may produce a signal ΔS which is twice the amplitude of a single set of capacitor plates, with none of the DC offset. This condition may be expressed mathematically as:ΔS=ΔSa−ΔSb=(Sa−Δ−Sa)−(Sb+Δ−Sb)=−2Δ  (1)wherein ΔSa and ΔSb are the changes in the signal from the left and right set of capacitor plates, respectively. Accordingly; monitoring the change in the differential output of the first set of capacitor plates 170a and 180a relative to the change in capacitance of the second set of capacitor plates 170b and 180b,may determine the magnitude and sign of accelerations applied perpendicular to the plane of the teeter-totter accelerometer 100.